How do I change my disc-brake pads? How do I avoid letting the fluid out?
The exact procedure varies a bit depending on the model of brake caliper/pads. Most disc brakes are now hydraulic. I have found that I have never had to so far let fluid out of or top up fluid in the hydraulic brakes! The procedure usually goes a bit like this:
TOOLS/INGREDIENTS: latex gloves, cloth, old toothbrush, white spirits, tyre remover tool, screwdriver, pliers(preferably needle nosed), allen key/wrench as appropriate depending on wheel removal/caliper
Always allow a bit of time for complications. ESPECIALLY if you have not done that particular bike/part before.
Collect tools/parts. Gloves on. Remove wheel (OR some calipers you remove one bolt and the caliper rotates away from the rotor like a motorbike brake).
Clean brake caliper outside. Using toothbrush + white spirits + cloth. WARNING: do not get dirty/oily fluid on rotor or braking surfaces!
OPTIONALLY: Use screwdriver to push brake pads apart as far as you can to each side (to push pistons back)
Pop out pin or whatever holds the brake pads in place. Remove pads from caliper. Inspect pads.
Clean inside of caliper.
Use plastic tool to push pistons back on each side.
Put new pads into holder. Slide them into caliper. Push in pin or whatever holds them in place and lock.
Put wheel back on, gently . . make sure rotor goes in between the brake pads in the caliper.
Pull the brake a few times. The first few times it will be soft and go all the way but it should stiffen up as the piston is pushed out and starts pushing the pads against the rotor.
At this stage you are almost done, the rotor could have got something dripped or splashed on it so clean the rotor with a completely fresh cloth/kitchen towell. If the rotor has any contamination on it brakes might be noisy and not as effective as they should be.
Test cycle. Hopefully job is done brake working fine. If not . . then . . CONTINGENCY/debugging TIME . .
Careful first few cycles out just in case . . .
Usually with disc brakes it is easy enough. I think they are easier to change than rim brake pads. But . . sometimes things get a bit tricky. Extra notes in case of complications:
Yes, you do need to remove the wheel. This gives access to put in and out pads and to push the pistons back. Some types of pads could be replaced without removing the wheel, but only if you can also clean and get pistons back.
Yes, gloves and cleaning are good to use/do. Cleaning helps keeps you and tools/bike clean and in long term helps prevent parts getting seized and wearing.
Before removing the pads you can use them to push the pistons back. Insert a screwdriver or different tool between the brake pads and push them to the side. This ensures the piston is not damaged by directly pushing with screwdriver.
I did once encounter a piston which was stuck: Hayes Sole Disk Brakes, adjusters stiff/seized?
In this case, remove the brake pads. Clean inside and outside with white spirits. Avoid getting dirty fluid on brake rotor or other braking surfaces. Do not clean/lubricate with oil or WD40 as it could damage any rubber/plastic parts of pistons. dot4 brake fluid could be used to lubricate (check is your hydraulic fluid mineral oil or dot4). After a while (in bad cases leave overnight for cleaning to soak in) gently (with plastic tool) but firmly push piston back.
I guess if that doesn't work then there might be air or excess of fluid in hydraulic system and some needs to be drained BUT I would really try and avoid draining any fluid!
For shimano there is a sort of cotter-pin. Bend or unbend the end to secure in place. For other pads there are different pins and small bolts or the caliper needs to be disconnected partly and angled away from rotor.
Inspect pads, I find usually pads go just a little bit too far worn in one place! When brake feels rougher or noisy I know I need to replace or inspect properly. Inspecting pads . . it is very hard to see how much pad is left without removing them. My shimano pads come off and the part closer to inside of the wheel is more worn, usually one end of a pad is almost completely down to the metal behind pad.
plastic tool = tyre remover or perhaps your old toothbrush :)
This step caused me trouble this morning! Shimano brake pads did not fit into the supplied pad holder they came with! They sort of fitted but hole for pin did not line up. I actually had to file the tops of the pads down to get them to fit into holder. Fun and interesting!? And educational.
First few cycles out, just watching out for anything funny. "bedding-in" brake pads? I dunno. Usually they just work immediately.
A different thing, more of a problem with rim brakes, in case where your old brakes were quite bad and now you have good brakes be careful as you might grab and squeeze brakes suddenly and actually stop!! Do a bit of MTB training and learn to feather brakes.